1. Field of Invention
The invention relates to electronic assemblies and more particularly to reducing heat conduction between devices mounted on a thermally conductive substrate.
2. Description of the Prior Art
It is common in semiconductor electronics and other electronic systems to mount multiple devices on a thermally conductive substrate. In some systems, some devices may be temperature sensitive and other devices may have varying power dissipation. For example, individual photo sensors or arrays of photo sensors may be mounted onto a ceramic substrate along with various heat producing devices such as amplifiers and microprocessors. Examples of photo sensor arrays include photodiode arrays and charge coupled device (CCD) arrays. Most photo-sensitive devices are affected by temperature. Temperature change impacts both accuracy and precision. For example, for charge coupled devices, dark current, a thermally generated noise source, increases with temperature. For photodiodes, diode leakage increases substantially with temperature. In addition, quantum efficiency changes with temperature and is a function of wavelength. In general, for arrays of photosensitive devices, temperature problems are made worse when temperature variations are nonuniform over the surface of the array. In addition, the problems are made worse when sensor temperature is a function of the measured signal.
One conventional technique for partially compensating for the effects of temperature is to use a heat sink. For example, see PCT/US90/05633. In PCT/US90/05633 a heat sink is attached directly to the back of a charge coupled device array and the combination is attached to a circuit board. Heat sinks are able to improve temperature uniformity over a surface and to keep the temperature close to the surrounding environment. However, heat sinks add size and weight to an assembly. In addition, as illustrated in PCT/US90/05633, a heat sink may require a special mounting arrangement to avoid interfering with optical sensors.
Another technique for partially compensating for the effects of temperature is to add temperature sensing devices. For example, see EP-B-0 148 703. In EP-B-0 148 703, temperature sensitive elements are used to directly compensate a voltage output signal. Adding temperature sensing devices can compensate for electronic effects of nonuniform temperature. However, temperature may also have mechanical effects (dimensional changes) and the temperature sensing devices cannot decrease the temperature or make the temperature uniform. In addition, the sensing devices may provide an additional source of heat.
Still another technique for compensating for the effects of temperature is to actively stabilize the temperature, for example with a Peltier element in contact with the temperature sensitive device. However, these devices are typically expensive and typically add size, weight and complexity to the overall system.